Short wavelength (ultraviolet, blue, green) coherent light sources are desirable for a number of applications, including high density (.apprxeq.1 Gbit/cm.sup.2) optical data storage, color image processing, such as in laser printers, submicron lithography and other high resolution laser processing steps for fabricating VLSI devices, satellite-to-submarine and other underwater optical communications, and spectroscopy and other optical measurements, such as interferometric gravity-wave detection and the like. In many of these applications, compact laser systems are desired, and in some, relatively high output powers (greater than about 100 mW) are required. Accordingly, considerable effort has been undertaken in recent years to develop more compact, short-wavelength coherent light sources to replace the low-power air-cooled gas lasers, such as argon ion and helium-cadmium lasers, that are presently the only practical sources which are available.
Because of their compactness, high electrical-to-optical conversion efficiency, wavelength tunability and rapid modulation capabilities, semiconductor laser diodes are being actively studied to discover whether shorter wavelengths can be generated. At present, AlGaInP lasers have the shortest practical limit of wavelength around 600 nm (yellow-orange). Potential wide band-gap semiconductor lasers made of AlGaN or other materials are being studied. At present, approaches to directly generating mid-infrared wavelengths are focused on rare-earth doped solid state lasers, such as thulium (Tm) and holmium (Ho) doped lasers.
Nonlinear optical processes, such as frequency doubling, also called second harmonic generation, and sum frequency mixing, are capable of converting red and near infrared light into ultraviolet, blue and green light. Accordingly, much development work has focused on using nonlinear frequency conversion techniques to generate ultraviolet, blue and green light directly from red and near infrared laser diodes. Direct frequency doubling of laser diode emission makes possible the extension of available diode laser wavelengths into the ultraviolet, blue and green regions of the spectrum, and represents at present the most feasible approach to developing a compact, efficient, high power coherent source in those spectral regions. However, in order for this approach to be successful and useful in practical applications, like those mentioned above, higher optical conversion efficiencies from the red and near infrared laser diode wavelengths to the desired ultraviolet, blue and green wavelengths are needed.
In general, higher optical conversion efficiencies are associated with a higher power density or intensity of the fundamental pump wavelength within the nonlinear optical material. Because of the relatively low powers available from most diode lasers, configurations using external resonators or channel waveguide structures have been preferred. For example, efficient frequency conversion is possible by coupling laser diode radiation into a passive external resonator of either a standing wave or unidirectional ring type that contains a bulk crystal of nonlinear optical material, such as potassium titanyl phosphate (KTiOPO.sub.4) or potassium niobate (KNbO.sub.3). The high circulating intensity that builds up in the crystal located within the resonator results in efficient frequency conversion of the laser diode radiation.
W. J. Kozlovsky, et al., in Applied Physics Letters 56(23), pages 2291-2292 (1990), describe frequency doubling of an 856 nm laser output from a ridge waveguide, single quantum well, graded index double heterostructure GaAlAs diode laser in a monolithic KNbO.sub.3 crystal ring resonator in order to generate 428 nm (blue) radiation. The ring resonator is a 7 mm long KNbO.sub.3 crystal with curved mirror end faces coated for high reflectivity at the fundamental wavelength and transitivity of the frequency doubled blue light and with a flat total internal reflection surface parallel to the mirror axes. The crystal resonator is placed on a thermoelectric cooler so that the temperature can be stabilized at 15.degree. C. for phase-matched second harmonic generation along the long direction of the ring path. In order to achieve efficient power buildup in the KNbO.sub.3 cavity and generation of stable blue output, the laser output frequency is locked to the cavity resonance using an elaborate electronic servo technique that superimposes a small RF current on the dc injection current to produce weak FM sidebands in the laser output and that uses a double-balanced mixer for phase sensitive detection of the optical-heterodyne-spectroscopy signal in the light reflected from the input surface of the resonator. Such a signal is zero when the carrier frequency coincides with the cavity resonance. The output signal of the mixer is amplified and coupled back to the laser injection current, so that the diode laser's output frequency tracks the resonance frequency of the KNbO.sub.3 cavity. Using such a servo technique, a 41 mW blue output (39% optical conversion frequency) was achieved. However, the technique requires a significant amount of electronics for it to work properly without amplitude noise. Elaborate temperature and electronic feedback controls for matching resonance frequencies are typical of external resonator systems. Besides being expensive and not very compact, in attempting to maintain stable operation, they usually introduce some wavelength jitter into the system.
J. T. Lin, in Lasers and Optronics, December 1990, pages 34-40, describes diode-pumped self-frequency-doubling (SFD) lasers using Nd.sub.x Y.sub.1-x Al.sub.3 (BO.sub.3).sub.4 (NYAB) crystals for the frequency doubling, and compares them against prior single-pass KTP, external resonator KNbO.sub.3 and channel waveguide LiNbO.sub.3 second harmonic generator configurations for diode lasers, as well as other frequency doubled laser systems. Up to 80 mW of output power (up to 8.0% efficiency) at 531 nm is achieved with NYAB compared to 40 mW of output power at 430 nm for external-resonator-type second harmonic generation of a 860 nm diode laser beam. Like diode-pumped solid-state lasers, these SFD laser systems are not particularly compact, so that a tradeoff between compactness and greater conversion efficiency must be made.
In U.S. Pat. No. 5,222,088 S. Amano discloses a blue laser formed by a semiconductor laser generating an excitation beam which is doubled by a crystal within a laser cavity. The second harmonic within the cavity is the desired wavelength. A cooling device is mounted on the frequency doubling crystal which stabilizes the frequency of the output beam.
In U.S. Pat. No. 4,809,291 Byer et al. disclose a diode laser which pumps neodymium lasing material within a laser cavity. The output of the neodymium material is doubled with a frequency doubling crystal and the output of the cavity is blue light having a wavelength of approximately 0.473 microns. Both the neodymium lasing material and the frequency doubling crystal are temperature stabilized with cooling elements.
Another approach for efficient frequency conversion is to use ion-diffused channel waveguides of nonlinear optical material, such as lithium niobate (LiNbO.sub.3) or potassium titanyl phosphate (KTiOPO.sub.4), to double the frequency of the laser diode emission. Doubling is relatively efficient if the waveguide is relatively long (greater than about 1 mm), but phase-matching of long frequency-doubling waveguides is critical, the available wavelength range is more limited, and fabrication tolerances are tight. Periodic poling can ease such requirements and increase efficiency. Another problem that arises when waveguide systems are used is that it is difficult to collimate and then focus the diode laser light to a diffraction-limited spot for efficient coupling into the waveguide, using conventional spherical lens systems. However, waveguide systems are compact.
C. J. Van der Poel, et al., in Applied Physics Letters 57(20), pages 2074-2076 (1990), describe second harmonic generation in periodically segmented KTiOPO.sub.4 (KTP) waveguide structures. The waveguide structures are formed in either flux-grown or hydrothermally grown KTP substrates by ion exchange through a Ti mask using various Rb/Tl/Ba nitrate molten salt baths. There are two segments per period, one segment being bulk KTP with a length l.sub.1 and a propagation constant mismatch .DELTA.k.sub.1, the other segment being an ion-exchanged KTP waveguide with a length l.sub.2 and a propagation constant mismatch .DELTA.k.sub.2, in which the phase-matching condition .DELTA.k.sub.1 l.sub.1 +.DELTA.k.sub.2 l.sub.2 =2.pi.M is met (M being an integer). Ferroelectric domain reversals in adjacent segments can also be included for higher conversion efficiencies. Efficient second harmonic outputs were observed from 0.38 .mu.m (deep purple) to 0.48 .mu.m (blue-green). W. P. Risk, in Applied Physics Letters 58 (1), pages 19-21 (1991), describes fabrication of optical waveguides in KTP crystals by an ion-exchange process involving a molten Rb/Ba nitrate bath. Second harmonic generation from titanium:sapphire laser light in the 900-1000 nm range was observed.
W. J. Kozlovsky et al., in Applied Physics Letters 58(23), pages 2291-2292 (1990) describe second harmonic generation of 428 nm (blue) light from an 856-nm current tuned laser coupled into a KNbO.sub.3 nonlinear, phase matched ring-resonator crystal. The crystal is mounted on a thermoelectric cooler for temperature control and stability. A closed loop servo technique is disclosed for locking a laser diode to the crystal.
A. Harada et al., in Applied Physics Letters 59(13), pages 1535-1537 (1991), describe second harmonic generation of 442 nm (blue) light from an 884-nm semiconductor laser using an organic crystal-cored nonlinear optical fiber coupled to the laser. The single transverse mode fundamental beam of the diode laser is collimated by a first objective lens and an anamorphic prism pair, and then focused into the fiber by a second objective lens. The fiber was formed by filling a hollow glass fiber by capillary action with the organic material (DMNP) melt, and then recrystallizing the polycrystals thus obtained by the BridgmanStockberger single crystal formation method in which the fiber is pulled out of a 105.degree. C. furnace. The fiber core diameter and length are 1.4 .mu.m and 5-15 mm, respectively. Output powers of 0.16 mW (about 1.6% conversion efficiency) were achieved. G. L. J. A. Rikken, et al., in Applied Physics Letters 58(5), pages 435-437 (1991), describe nonlinear optical effects in side chain copolymers with methylmethacrylate.
Efficient frequency conversion also requires good spectral mode properties of the diode lasers. In general, the laser wavelength needs to be stabilized to within 10 nm of the optimum wavelength for frequency doubling. The diode source must meet these wavelength specifications in the presence of feedback and temperature variations. Distributed Bragg reflector (DBR) lasers and distributed feedback (DFB) lasers use a grating to provide feedback and are known to have increased spectral and temperature stability compared to other laser diode sources.
An object of the invention is to provide an efficient, short wavelength (ultraviolet, blue or green), compact laser source.